1. Field of the Invention
This invention relates to a corrosion-resistant member having a resistance to a plasma of a halogen based corrosive gas, a wafer-supporting member using the same, and a method of manufacturing the corrosion resistant member.
2. Description of Related Art
Processes requiring a chemical reaction become increased as fine machining more proceeds with the increase of a memory capacity in super LSI. Especially, in a semiconductor manufacturing apparatus requiring a super clean condition, a halogen based corrosive gas such as chlorine based gas, fluorine based gas or the like is used as a gas for deposition, etching or cleaning.
For example, when a heat CVD device or the like is used as a heating device in the semiconductor manufacturing apparatus for heating at a state of contacting with these corrosive gases, a halogen based corrosive gas such as ClF3, NF3, CF4, HF, HCl or the like is used as a gas for cleaning the semiconductor after deposition. And also, a halogen based corrosive gas such as WF6, SiH2Cl2 or the like is used as a gas for forming a film at the deposition stage.
The inventors have disclosed in JP-A-5-251365 that an aluminum nitride sintered body provided on its surface with an aluminum fluoride layer has a high corrosion resistance against a plasma of the above halogen based corrosive gas. Namely, even when the aluminum nitride sintered body is exposed to, for example, ClF3 gas for one hour, no change of the surface state is observed.
Furthermore, the inventors have disclosed that an aluminum fluoride film is formed on the surface of the aluminum nitride sintered body by a gas phase method such as a CVD method or the like (JP-A-5-251365). And also, JP-A-7-270353 discloses that in order to prevent the occurrence of corrosion in a surface of an electrostatic chuck for a semiconductor wafer, the surface of the electrostatic chuck is previously subjected to a surface treatment replacing with fluorine to form AlF3 on the surface of the electrostatic chuck.
However, the inventors have made various studies and confirmed that when the aluminum nitride-base ceramic body is exposed to the halogen based corrosive gas such as ClF3 or the like in a high temperature range, particularly above 500xc2x0 C., the corrosion of the ceramic is promoted in accordance with the exposure condition to create particles.
It is, therefore, an object of the invention to provide a corrosion-resistant member capable of preventing the occurrence of corrosion and hence the occurrence of particles even when it is exposed to a plasma of a halogen based corrosive gas over a wide temperature range of from low temperature to high temperature, especially a high temperature range of not lower than 500xc2x0 C.
According to a first aspect of the invention, there is the provision of a corrosion-resistant member having a resistance to plasma of a halogen based corrosive gas, comprising a main body and a corrosion-resistant layer formed on a surface of the main body, in which the corrosion-resistant layer contains a fluoride of at least one element selected from the group consisting of rare earth elements and alkaline earth elements.
According to a second aspect of the invention, there is the provision of a wafer-supporting member exposed to a plasma of a halogen based corrosive gas, comprising a main body and a corrosion-resistant layer formed on a surface of the main body, in which the corrosion-resistant layer contains a fluoride of at least one element selected from the group consisting of rare earth elements and alkaline earth elements.
In general, a corrosion-resistant ceramic indicates an ion reactivity for an acid or alkaline solution. In the invention, however, the ion reactivity is not noticed, but a reactivity for a redox reaction through a halogen in a dry gas or plasma is noticed.
The inventors have investigated a reason why the corrosion proceeds in the aluminum nitride-base ceramic body provided with a passive coating of aluminum fluoride or the like when being particularly exposed to the plasma of the halogen based corrosive gas in the high temperature range. As a result, in the corrosion-resistant member wherein the corrosion is proceeding, the passive coating made of aluminum fluoride substantially disappears from the surface of the ceramic, while aluminum nitride-base particles existing under the passive coating are corroded and also a grain boundary phase existing between the aluminum nitride particles is subjected to the corrosion.
Although the reason of causing such a corrosion is not clear, it is considered that since a vapor pressure of AlF3 is relatively high and a temperature of rendering the vapor pressure of AlF3 0.001 Torr is about 695xc2x0 C., a process of evaporation of AlF3 proceeds in the high temperature range and hence the corrosion of aluminum nitride particles begins from the vicinity of the region where the passive coating made of AlF3 disappears.
For example, the temperature of rendering the vapor pressure 0.001 Torr is 1066xc2x0 C. in MgF2, 1195xc2x0 C. in CaF2, 1233xc2x0 C. in SrF2, 1065xc2x0 C. in BaF2, 975xc2x0 C. in ScF3, 1100xc2x0 C. in PrF3, 1134xc2x0 C. in EuF2, and 695xc2x0 C. in AlF3.
In order to solve the above problem, the inventors made further studies and found that when aluminum nitride ceramic body containing a specific sintering aid is corroded under violent corrosion conditions, the proceeding of corrosion stops at a certain time and a novel passive coating having a considerably excellent corrosion resistance is unexpectedly formed on the surface of the ceramic body. Surprisingly, this coating has an extremely high corrosion resistance to the plasma of the halogen based corrosive gas above 500xc2x0 C.
According to a third aspect of the invention, there is the provision of a method of manufacturing a corrosion-resistant member, which comprises the steps of: firing a powder containing 100 parts by weight of aluminum nitride and not less than 100 ppm but not more than 60 parts by weight of at least one element selected from the group consisting of rare earth elements and alkaline earth elements to produce a dense aluminum nitride ceramic sintered body; and then holding the sintered body in a plasma of a fluorine containing gas at a temperature of 500xc2x0 C.-1000xc2x0 C. to form a corrosion-resistant layer.
According to a fourth aspect of the invention, there is the provision of a method of manufacturing a corrosion-resistant member, which comprises the steps of: providing an aluminum nitride ceramic main body comprising aluminum nitride-base grains and grain boundary phase existing in grain boundaries of the aluminum nitride-base grains and containing in its grain boundary at least one element selected from the group consisting of rare earth elements and alkaline earth elements; and then holding the main body in a plasma of a fluorine containing gas at a temperature of 500xc2x0 C.-1000xc2x0 C. to form a corrosion-resistant layer.
In the sintering of aluminum nitride, a sintering aid such as yttria or the like may be added in order to promote the sintering process and raise a thermal conductivity and a mechanical strength of the resulting sintered body. After the completion of the sintering, a great amount of such a sintering aid is existent in the grain boundary phase of the aluminum nitride grains. In the conventional knowledge, it is considered that when aluminum nitride ceramic body containing the sintering aid is exposed to the plasma of the halogen based corrosive gas, fluorine radicals or the like diffuse along the grain boundary phase to change the volume of the grain boundary and hence aluminum nitride grains are detached to prematurely cause the corrosion.
However, it is surprisingly discovered that when such an aluminum nitride ceramic body is exposed to the plasma of the halogen corrosive gas at a high output under a specified sever condition of high temperature, the passive coating as mentioned above is formed.
The passive coating contains a fluoride of a rare earth element or an alkaline earth element as a prominent ingredient. It is considered that such an ingredient has the same high corrosion resistance as the aluminum fluoride and is difficult to evaporate even at a higher temperature as compared with the aluminum fluoride, which contributes to considerably improve the corrosion resistance of the corrosion-resistant member according to the invention against the plasma of the halogen based corrosive gas.
The inventors have made further studies and found that even when a main body made of material other than the aluminum nitride ceramic body is used as a main body of the corrosion-resistant member and a corrosion-resistant layer made of a fluoride of at least one element selected from the group consisting of rare earth elements and alkaline earth elements is formed on a surface of the main body by a gas phase method or the like, a high corrosion resistance against the halogen based corrosive gas can be obtained at an extremely high temperature range over a long time.
In this case, the corrosion-resistant layer is particularly preferable to be a film made of the fluoride, which can provide substantially a homogeneous protection over a wide range.
Explaining in more detail, the stability against fluorine based gas as well as chlorine based gas and bromine based gas having an electronegativity lower than that of fluorine can be ensured by forming a compound layer of fluorine having a high electronegativity. And also, the stability at a high temperature can be obtained by using the fluoride of a rare earth element or an alkaline earth element having a low vapor pressure.
As the fluoride, the aforementioned products are preferable, and magnesium fluoride is particularly preferable.
Further, the main body is preferable to be made of a material selected from the group consisting of a metal aluminum, a metal silicon, a thermal-resistant alloy, a silicon nitride-base ceramic, a silicon carbide base ceramic, alumina, boron carbide and silicon oxide.
Furthermore, the inventors have further studied and found that the corrosion-resistant layer of the fluoride can be produced by forming a surface layer made of a compound of at least one element selected from the group consisting of rare earth elements and alkaline earth elements on the surface of the main body and holding such a surface layer in a plasma of fluorine containing gas at a temperatures of 500-1000xc2x0 C. Thus, the corrosion-resistant layer of the fluoride can be produced more surely and easily.
The material of the surface layer is not particularly limited, but it is preferable to be a single oxide of a rare earth element and aluminum or an oxide of two or more metals. The surface layer is more preferable to be made of one or more oxides selected from the group consisting of Y2O3xe2x80x94Al2O3 two-component system oxide and Y3Al5O12.
The invention will be explained more concretely below.
It has been found that the corrosion-resistant member according to the invention is stable against a plasma of a halogen based corrosive gas such as ClF3 gas, NF3 gas, CF4 gas or WF6 gas as well as Cl2 gas, BCl3 gas or the like.
When the corrosion-resistant member according to the invention is used as a wafer-supporting member, particularly as a susceptor for supporting a semiconductor wafer, there can be provided a structural part being stable against a cleaning gas or an etching gas and also the generation of particles or contamination resulting in the reject of the semiconductor can be prevented over a long period. Thus, the invention can be first applied even to the manufacture of a high integrated semiconductor such as DRAM, 4M or the like.
The invention is very useful irrespectively of the film-forming and cleaning for the semiconductor when the invention is applied to a susceptor generating heat by heating through an infrared lamp, a ceramic heater for heating a semiconductor, a susceptor disposed on a heating surface of the ceramic heater, a susceptor embedding an electrode for an electrostatic chuck therein, a susceptor embedding an electrode for an electrostatic chuck and a heating resistor therein, a susceptor embedding an electrode for generating a high frequency plasma therein, or a susceptor embedding an electrode for generating a high frequency plasma and a heating resistor therein. Because it is desired to develop a useful material as these parts may be exposed to a halogen based corrosive gas at a high temperature of not lower than 500xc2x0 C.
Further, the corrosion-resistant member according to the invention can be used as a substrate for the semiconductor manufacturing apparatus such as a dummy wafer, a shadow ring, a tube for generating a high frequency plasma, a dome for generating a high frequency plasma, a high frequency transmitting window, an infrared transmitting window, a lift pin for supporting a semiconductor wafer, a shower plate or the like.
Since the metal member embedded in the susceptor is usually fired together with the aluminum nitride powder, it is preferable to be made of a metal having a high melting point. As such a high melting point metal, mention may be made of tantalum, tungsten, molybdenum, platinum, rhenium, hafnium, and an alloy thereof. From a viewpoint of the prevention of a semiconductor from contamination, tantalum, tungsten, molybdenum, platinum and alloys thereof are more preferable.
And also, the cleaning gas or etching gas made of the halogen based corrosive gas is used in a chemical industry other than the semiconductor manufacturing industry, so that the corrosion-resistant member according to the invention is useful even in the field of the chemical industry.
As the rare earth element, Y, Yb, Ce, Pr and Eu are particularly preferable, and as the alkaline earth element, Mg, Ca, Sr, and Ba are preferable.
Among these elements, an element having an ion radius of not less than 0.9 angstrom is particularly preferable. The ion radius is an ion radius in six coordination defined through a method described by R. D. Shannon and C. T. Prestwitz, xe2x80x9cActa Cryst.xe2x80x9d B25, p925 (1969). As such an ion radius, there are La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Eu2+, Gd3+, Tb3+, Dy3+, Ca2+, Sr2+, Ba2+and Ra2+.
In the above manufacturing methods according to the invention, powder obtained by a direct nitriding method can be used as a powdery aluminum nitride raw material, and also powder obtained by a reductive nitriding method may be used.
The rare earth element and/or the alkaline earth element may be added to the powdery aluminum nitride raw material in various forms. For example, powder of a single body or compound of the rare earth element and/or the alkaline earth element may be added to the aluminum nitride raw material powder. When the amount added as the sintering aid exceeds 60 parts by weight, the thermal conductivity of the resulting aluminum nitride ceramic body decreases to less than 60 W/(mxc2x7K) and tends to be not practical.
As the compound of the rare earth element or the alkaline earth element, an oxide of a rare earth element is generally most available. The compound of the rare earth element or the alkaline earth element such as nitrate, sulfate, alkoxide, fluoride or the like is dissolved in a proper solvent capable of dissolving such a compound to obtain a solution, which may be added to the powdery aluminum nitride raw material. Thus, the rare earth element is easy to be uniformly dispersed into any portions of the sintered body.
In the preparation step, the powdery aluminum nitride raw material is dispersed into the solvent, to which may be added the compound of the rare earth element and/or the alkaline earth element in the form of oxide powder or solution. The mixing may be carried out by simple stirring, but if it is required to pulverize aggregates in the powdery raw material, a mixing and pulverizing machine such as pot mill, trommel, attrition mill or the like may be used. In the case of using an additive soluble in the solvent for pulverization, the time of conducting the mixing and pulverizing step may be a shortest time required for the pulverization of the powder. Moreover, a binder such as polyvinyl alcohol or the like may be added.
A spray drying process is preferable for the step of drying the solvent for pulverization. And also, it is preferable to adjust a grain size of a dried powder through a sieve after the conduction of a vacuum drying process.
In the powder shaping step, a mold pressing process may be employed for the manufacture of a disc-shaped body. In this case, a shaping pressure is preferable to be not less than 100 kgf/cm2, but is not particularly limited if it is possible to maintain the shaping. Further, the shaping material may be filled in a hot press die in the form of powder. When the binder is added to the shaped body, the degreasing may be carried out at a temperature of 200-800xc2x0 C. in an oxidizing atmosphere prior to the firing.
Then, the shaped body is preferably fired by a hot pressing process, a hot isostatic pressing process or the like. The pressure in the hot pressing process or the hot isostatic pressing process is preferable to be not less than 50 kgf/cm2, more particularly not less than 200 kgf/cm2. The upper limit of the pressure is not particularly restricted, but is practically not more than 1000 kgf/cm2 in order to prevent the damage of the furnace tool such as a mold or the like.
Further, it is preferable to raise the temperature at a temperature rising rate of not less than 50xc2x0 C./hour but not more than 1500xc2x0 C./hour up to a maximum temperature in the firing. The maximum temperature is preferable to be 1700-2300xc2x0 C. When the maximum temperature exceeds 2300xc2x0 C., aluminum nitride begins to decompose. When the maximum temperature is lower than 1700xc2x0 C., the effective grain growth is restrained.
When the aluminum nitride-base ceramic body as the sintered body or the like is held in a plasma of a fluorine-containing gas at 500-1000xc2x0 C., it is rather preferable to adopt a severe condition. For example, a temperature is preferable to be 600-800xc2x0 C., and a plasma power is preferable to be not less than 500 W. A formation stage of the corrosion-resistant layer at this process will be further explained below.
The method of producing the film made of the fluoride on the main body is not particularly limited so long as defects and pin-holes are not formed in the film. However, if the shape of the main body is complicated or large, an ion plating method is preferable, while if it is a simple shape such as a cover plate or a small type, a sputtering process is preferable. Moreover, when the coating is carried out by these method, it is favorable that the surface of the main body is previously cleaned by a method such as a reverse sputtering or the like. And also, a chemical vapor growth method, a spraying method and a powder coating plus heat treatment may be adopted.
The thickness of the corrosion-resistant layer is not particularly limited so long as defects such as cracking, peeling or the like are not generated. If the corrosion-resistant layer is too thick, fracture, cracking or the like is liable to be caused in the corrosion-resistant layer by heat stress based on the difference of thermal expansion between the substrate and the corrosion-resistant layer, so that the thickness is preferable to be not more than 10 xcexcm, more particularly not more than 4 xcexcm.
In order not to form the pin-hole in the corrosion-resistant layer, the thickness is preferable to be not less than 0.2 xcexcm, more particularly not less than 1 xcexcm.